1. Field of the Technology
The technology relates to an image forming apparatus capable of controlling a charging potential of a photoconductor and a developing bias voltage in an electrophotographic process, and having a function of adjusting a density of a formed image.
2. Description of the Related Art
When an image with a low toner coverage ratio, namely, an image having few parts where toner is adhered out of an entire area of a print image, is printed continuously, it is known that a granular “fog” (phenomenon that the toner is adhered to a white background part where the toner is not supposed to be adhered) is generated eventually. Although a cause of this type of fog is not clarified, it is empirically known that after printing at a low toner coverage ratio is continued to some extent, the fog is generated. From this fact, it is estimated that when the same toner is retained in a developing unit, a kind of deterioration occurs to the toner, thus causing the fog.
Generation of the fog is not preferable in terms of image quality. Therefore, a technique of preventing the fog is proposed, in such a way that the fog on the photoconductor caused by the deterioration of the toner is detected by an optical sensor, and when the fog is generated, a toner image is formed on both ends in a direction of a photoconductor rotation axis outside of an image area, and the toner is forcibly discharged to replace the toner (for example see Japanese Unexamined Patent Publication No. 2006-243115).
However, since the fog is a phenomenon in which the toner is extremely thinly applied to a non-image area, it is difficult to stably detect the fog with accuracy. In addition, it may be preferable to predict the generation of the fog and then cope with the fog, rather than coping with the fog after actually it is generated.
As a result of earnest efforts to study on a condition of allowing the fog to be generated after printing at a low toner coverage ratio, inventors of the technology find a point that there is a correlation between the charging potential of the photoconductor and the generation of the fog. Namely, it is found that the larger an absolute value of the charging potential of the photoconductor is, the more easily the fog is generated. Moreover, it is found that when the toner is retained for a long period in the developing unit, the density of the image hardly appears. As a result, it is found that an image density adjustment process control executed so as to stabilize the image density makes an absolute value of the charging potential of the photoconductor large, thus leading to a circumstance where the fog is easily generated.
In addition, it is confirmed that there is a correlation between a use period of the photoconductor and the generation of the fog. Namely, it is confirmed that when the photoconductor is new, the fog is hardly generated, and as the use period is elapsed, the fog is easily generated.
Neither deterioration of the image density nor the fog is preferable, in terms of the image quality. The deterioration of the image density and the fog must be suppressed, so as not to be recognized by a user. However, in a case where the charging potential is controlled to stabilize the image density when the printing at a low toner coverage ratio is continued as described above, the fog is easily generated. Accordingly, there is desired a technique capable of accurately predicting or determining a condition where the fog is easily generated, and a technique capable of appropriately coping with such a condition.